Warmup timer is not working on 4.20

Hello, I have updated to the version 4.20 and suddenly all my spawners are broken, the warmup timer is ignored, I have tested this on spigot 1.8.8 and 1.12.1

How to replicate?

1. Create a mob
2. Create a spawner
3. Set the warmup to 30
4. Kill the mob

The mob will spawn immediately ignoring the warmup